The present invention is a one-step process for the production of monosilanes from the high-boiling residue resulting from the reaction of organochlorides with silicon metalloid in a process typically referred to as the "direct process." The process comprises contacting a mixture comprising the high-boiling residue, an organotrichlorosilane, and hydrogen chloride with a catalytic amount of a catalyst composition effective in promoting the formation of monosilanes from the high-boiling residue. A preferred catalyst composition for use in the present process comprises aluminum trichloride, at least a portion of which may be formed in situ during conduct of the direct process and isolation of the high-boiling residue.
In the preparation of organochlorosilanes by the direct process a complex mixture is formed which is typically distilled to separate monosilanes from other components present in the mixture. For example, in the "direct process," in addition to the monosilanes which in the case of the chloromethylsilanes include dimethyldichlorosilane, methyltrichlorosilane, and trimethylchlorosilane there is obtained a residue which boils above the organochlorosilanes, that is above about 70.degree. C. This residue is hereinafter referred to as "high-boiling residue."
The "direct process" is well described in the patent literature, for example, in Rochow, U.S. Pat. No. 2,380,995, issued Aug. 7, 1945, and Barry et al., U.S. Pat. No. 2,488,487, issued Nov. 15, 1949. The residue remaining after distillation overhead of the monosilanes is a complex mixture comprising higher boiling silicon containing compounds which have, for example, SiSi, SiOSi, and SiCSi linkages in the molecules. The residue may also contain silicon particulates and metals or compounds thereof. Typical high-boiling residues obtained from distillation of product from the direct process are described, for example, in Mohler et al., U.S. Pat. No. 2,598,435, issued May 27, 1952, and Barry et al., U.S. Pat. No. 2,681,355, issued Jun. 15, 1954.
In current commercial operations for performing the direct process, the high-boiling residue can constitute as much as five weight percent of the resultant product. Therefore, it is desirable to convert the high-boiling residue into commercially desirable products to both reduce waste disposal and to improve raw material utilization.
Ritzer et al., U.S. Pat. No. 4,393,229, describe a process for converting alkyl-rich disilanes in a residue obtained from the manufacture of alkylhalosilanes to halogen-rich polysilanes. The process comprises treating an alkyl-rich disilane-containing residue with an alkyltrihalosilane or silicon tetrahalide in the presence of a catalyst and a catalytic amount of a hydrosilane reaction promoter at an elevated temperature. Ritzer et al. teach aluminum trichloride as a useful catalyst in their process when used with a hydrosilane promoter. Ritzer et al. further teach that the resulting halogen-rich polysilanes can, in a separate step, be cleaved to form monosilanes.
Bokerman et al., U.S. Pat. No. 5,175,329, describe a process for the production of organosilanes from the high-boiling residue resulting from the direct process that results in a net consumption of organotrichlorosilane. In the described process the high-boiling residue is contacted with an organotrichlorosilane and hydrogen gas in the presence of both a hydrogenation catalyst and a redistribution catalyst.
Ferguson et al., U.S. Pat. No. 5,430,168, teach a process comprising forming a mixture comprising an organotrihalosilane and a high-boiling residue in the presence of hydrogen gas and a catalytic amount of aluminum trichloride. The process results in a net consumption of the organotrihalosilane and the formation of monosilanes.
Barry et al., U.S. Pat. No. 2,681,355, teach that a high-boiling residue resulting from the direct process can be contacted with hydrogen chloride at elevated temperatures to form methylchlorosilanes.
Bluestein, U.S. Pat. No. 2,709,176, teaches heating an organohalogenpolysilane containing a silicon-silicon linkage at an elevated temperature in the presence of a tertiary organic amine and hydrogen halide to effect cleavage of the silicon-silicon bond and form organohalogenmonosilanes.
Chadwick et al., U.S. Pat. No. 5,292,912, teach a process comprising contacting a high-boiling residue from the direct process with hydrogen chloride at a temperature within a range of about 250.degree. C. to 1000.degree. C. in the presence of a catalyst selected from a group consisting of activated carbon, platinum supported on alumina, zeolite, aluminum trichloride, and aluminum trichloride supported on a support selected from a group consisting of carbon, alumina, and silica.
An object of the present invention is to provide a one-step process where the high-boiling residue from a direct process for producing organohalosilanes can be converted into commercially useful monosilanes. An advantage of the process is that it can be run as a one-step process with no need to isolate more chlorinated intermediates, such as chlorodisilanes, and further treat to produce monosilanes. Another advantage of the process is that it can convert both disilanes and other silicon containing species comprising the high-boiling residue, such as silmethylenes and silalkylenes, to monosilanes. Typically, the monosilane product of the present process comprises almost exclusively methyltrichlorosilane and tetrachlorosilane.